Apparatus and method for operating a forced flow once-through vapor generator



Jan. 5, 1965 E. KOCHEY, JR 3,

APPARATUS AND METHOD FOR OPERATING A FORCED FLOW ONCE-THROUGH VAPORGENERATOR 2 Sheets-Sheet 1 Filed Nov. 20, 1962 54" X ZQ FINISHINGSUPERHEATER AUXILIARY VAPOR SOURCE PRIMARY SUPERHEATER DE'AERATORecouomzen PREHEATER INVENTORZ EDWARD L..KOCHEY JR. BY wafi AGENT Jan. 5,1965 E. L. KocI-IEY, JR 3,164,134

APPARATUS AND METHOD FOR OPERATING A FORCED FLOW ONCE-THROUGH VAPORGENERATOR 2 Sheets-Sheet 2 Filed Nov. 20, 1962 I FINISHING ./I-SUPERHEATER PRIMARY SUPERHEATER ECONOMIZER I l l I 1 l l l I I lDE'AERATER FIG. 2

INVENTOR. EDWARD L. KOCHEY JR. 3%?

AGENT United States Patent 3,164,134 APPARATUS AND METHGD FOR OPERATINGA FORED FLOW ONCE-THROUGH VAPOR GEN- ERATOR Edward L. Koehey, Jr.,Colebrook, Conn., assignor to Combustion Engineering, Inc., Windsor,Conn., a corporation of Delaware Filed Nov. 20, 1962, Ser. No. 238,887 9Claims. (Ci. 122-406) The invention relates in general to a forced flowmodified once-through vapor generating power plant and more particularlyto an apparatus and method for starting up the vapor generator andturbine associated therewith.

It is known in starting up forced flow once-through vapor generators toseparate the vapor superheating and turbine portion from the vaporgenerating portion by a so-called boiler throttling valve. Suchseparation permits the starting up of the steam generating portionindependent and simultaneously with the start-up operation of theturbine with, for instance, vapor being used from an auxiliary sourcefor the latter operation. Such separation also permits the starting upof the vapor generator with only vapor entering the superheater portionwhether such vapor originates from an auxiliary source or is obtained bythrottling and expansion of the high pressure fluid heated in the vaporgenerating portion.

In large vapor generators generally two or more superheating sectionsare employed. In these instances the boiler throttling valve ispreferably located downstream of the primary superheater instead ofupstream thereof. This permits heating of the working fluid in the vaporgenerating portion to a higher temperature such as 800 F. before thisfluid is expanded to the lower pressure region of the intermediate orfinal superheater section during a later phase in the start-upprocedure. In forced flow modified once-through vapor generators arecirculation circuit is used which recirculates the working fluidaround the vapor generating section for the purpose of speedily raisingthe pressure and temperature thereof to the desired operating values. Toobtain accurate measurements of the temperature of the fluid at a pointbetween the primary superheater outlet and the boiler throttling valve,some flow of this fluid must be established away from such point to alower pressure region.

It is accordingly a primary object of the invention to establish thisflow Without destroying the advantages of the recirculation circuit andWithout necessitating the feeding of working fluid to the vaporgenerator or the draining of such fluid from the vapor generating systemduring a major phase of the start-up operation.

Other objects and advantages will become apparent as the description ofillustrative embodiments of the invention proceeds. The novel featureswhich are considered characteristic of the invention are set forth withparticularity in the appended claims. These claims as Well as thefollowing description of the invention will best be understood when readin conjunction with the accompanying drawing in which:

FIG. 1 is a representation of a vapor power plant system in the form ofa flow diagram incorporating the features of the present invention asemployed in connection with a vapor generator using vapor from anauxiliary source for starting up the turbine associated with the vaporgenerator.

FIG. 2 is a flow diagram representing a vapor power.

plant system similar to that depicted in FIG. 1, however illustratingthe invention in connection with a vapor generator wherein the vapor forcooling the superheater and starting the turbine is produced byexpanding and flashing the high pressure working liquid.

Referring now to the drawing in which like reference characters are usedthroughout to designate like elements, the diagrammatic representationof FIG. 1 shows a forced flow once-through vapor generator 10. A feedpump 12 is organized to supply working fluid to vapor generator 10 fromasource such as de-aerator 14 by Way of conduit 16 and via a fluidpreheater 18. To control or shut-ofl the flow of working fluid a feedvalve 24) is provided in conduit 16. The working fluid passes througheconomizer 22, conduit 23, vapor generating section 24, conduit 25,primary superheater 26, conduit 27, finishing superheater 28 and conduit29 to a point of use such as vapor turbine-30. A valve 31 is provided inconduit 29 to shut-off the flow of the working fluid to turbine 30.After having given up a major portion of its thermal energy, the vaporis condensed in condenser 32 and the condensate returned to de-aerator14 through conduit 33 via condenser pump 34 and heater 35.

Fuel and air for combustion are supplied to vapor generator 10 by way ofburner 36 in any conventional manner. The hot combustion gases producedby the burning of the fuel pass in heat exchange relation overthe heatabsorbing surfaces of the vapor. generating section 24, superheatingsections 26 and 28 and economizer 22. Oth er conventional means ofsupplying heat to the vapor generator may be used in connection with theinvention.

In accordance with the invention a source 38 of auxili ary vapor isprovided for supplying vapor during the startup operation to primarysuperheater 26 by way of conduit 40 including valve 42. An importantelement of the inventive combination herein disclosed is the provisionof a recirculating conduit 44 including valves 4-6 and recirculatingpump 48 for the recirculation of working fluid from the outlet of vaporgenerating section 24 to the inlet thereof. In addition an overflowconduit 50' is provided;

including valves 51 and 52 which connects the outlet of primarysuperheater 26 to a point of low pressure such as the hot well ofcondenser 32 or de-aerator 14; Furthermore the invention contemplatesthe use of a by-pass conduit 53 including valve 54 for lay-passing theturbine during an early phase of the start-up operation. Anotherimportant element of the inventive combination is the provision inconduit 25 of a shut-off valve 55 including a throttling valve 56arranged in conduit 57 which bypasses valve 55. The purpose of bypassvalve 56 is to obtain throttling with high pressure drop and/or smallflow quantities, while the main shut-off valve 55 is designed forrelatively low pressure drops and large flow quantities.

The forced flow modified once-through vapor generator is started upaccording to the invention with the following general considerations inmind. First as earlier mentioned herein the vapor generating power plantis divided into two main portions. These are separated by a socalledboiler throttling valve 55. With this valve closed it' is possible byuse of vapor obtained from an auxiliary source 38 and superheated bypassing it through superheaters 26 and 28, to warm up and roll theturbine long before any vapor has been generated in the vapor generatingportion 24 of the generator 10. During the. time the turbine is thusbeing prepared for full operation with auxiliary vapor, the vaporgenerator is also being brought up to temperature and pressure with avaporizable fluid which fluid may have previously been de-aerated alsoby the use of vapor obtained from auxiliary source 38.

Accordingly the starting up operation of th e forcedflow once-throughvapor powerplant commences with the closing of the boiler throttlingvalve 55 and bypass valve 56, and the opening of boiler extraction oroverflow valves 51 and 52. Vaporizable fluid is pumped from thedeaerator through economizer 22 and vapor generating section2l by meansof feed pump 12 via feed valve 29. This fluid is first being dischargedto a point of lower pressure such as condenser 32 by way of extractionor overflow line Stl and extraction valves Bland 52, while the unit isbeing fired up. The heated fluid is thus discharged until the socalledclean-up operationof the vapor generator is completed, i.e., until theworking fluid has been purged of decontaminants such as iron oxide whichmay have been present in the Working fluid. At such time feeding of theworking fluid is discontinued by closing feed valve Eli and also byclosing extraction valve .51, with the latter however being set to openautomatically at a predetermined higher pressure. This is necessary inorder to allow for overflow due to expansion of the fluid as the fluidis being heated in the'vapor generating section 24.

A second consideration of the present invention is the provision forprotecting the tube lined walls 24 of th furnace from overheating whilethe feed valve 25 is closed. This is accomplished by recirculating theWorking fluid from the outlet of the vapor generating section to theinlet thereof by way of conduit 44 and valve 46 and by means ofrecirculating pump 48. Recirculation of the workingfluid thus serves twopurposes. First it provides for eflicient cooling of the tubular furnacesurface exposed to radiant heat, thereby permitting a faster increase ofthe firing r ate. And second, such recirculation simultaneously resultsin a quicker heating of the fluid since none of the heated fluid isbeing'discharged to waste. Such discharge and waste of heat would beunavoidable without recirculation since a minimum velocity of theworking fluid must be-maintainedin the furnace tubes to preventoverheatingthere'of.

While the vapor generating portion of the unit is thus prepared for fulloperation in a minimum of time by virtue of the recirculation circuitthe turbine is warmed up at the same time and rolled by being suppliedwith auxiliary vapor from source 38 with this vapor having beensuperheated in superheaters 26 and 28. When the fluid contained in vaporgenerating section 24 and continually being recirculated thereabout hasreached a temperature and pressure exceeding the temperature andpressure in the superheater 26 by a predetermined desired amount, valve56 in bypass 54- is opened with the high temperature fluid expandinginto primary'superheater 26, which; leads to the. third and mostimportant consideration of the inventive combination.

This third consideration hinges upon the fact that in V order toaccurately measure the temperature of the Working fluid in the conduit27 upstream of valves 55 and 56 a continuous flow must be establishedout of conduit 27. It is of primary importance that the pressure andtemperature in conduit 2'7be accurately obtained'since the difference inpressure and tempertaure between the fluid or vapor in vapor generatingsection ZAand that in superheater 26 is chosen to be of such an amountthat the throttling action taking place in valve 55 results in aflashing of vapor without any appreciable amount of liquid entering theprimary superheater 26. While flow of the working fluid out of conduit27 could be accomplished by way of conduit 56 and valve Sisuch flow atthis time would not be desirable for several reasons of economy. a

First, such flow would lower the eihciency or" operation by discharginga portion of the heated high pressure fluid to a region of low pressuresuch as the hot well of condenser 32 or the de-aerator 14;. Second, alarge portion of the heat containedin'the discharged fluid wouldbej-lost I if conducted to the condenser. Such heat as well as pressurewould have to be added again, to the relatively cold working fluid thatwould have to be supplied byway of feed conduit-Hand feed pump 12. Andthird, the time for bringing the working fluid that is recirculatingaround furnace walls 24 to the desired high temperature and pressurewould be unduly prolonged,- thereby partially defeaing the beneficialresult of recirculating conduit er.

To avoid the above'disadvantages the invention provides a return conduitas leading from conduit 27 to the recircula-ting conduit 44 or to theinlet of vapor generating section 24. A continuous flow of fluid isthereby established out of the primary superheater 26 withoutdischarging this flow to waste, or appreciably lowering the pressurethereof, or requiring replenishment of hot high pressure fluid byrelatively cool working fluid which must be supplied by feed pump 18, orprolonging the time required to heat the working fluid that isrecirculating through furnace Wall 24 and to raise it to operatingpressure. Thus a continuous flow of fluid is being provided leavingconduit 27. The pressure and temperature of this fluid can convenientlyand accurately be measured by means of devices 64 and 66 respectivelyand continuously be observed while the vapor generator is prepared forfull operation. When the pressure and temperature have reached valueswhich assure a complete flashing into vapor of the working fluid when itis throttled down to the pressure prevailing in finishing superheater28, valve 56 is cracked open either manually or by the action of controlelement 68 receiving temperature and/or pressure impulses from measuringdevices 66 and d4. Simultaneously with the opening of bypass valve 56feed valve 20 is gradually being opened to re-establish flow of workingfluid into the vapor generator. Also the flow of auxiliary vapor isproportionally reduced by controlling valve 42. The feeding ofrelatively cool working fluid into economizer 22 and vapor generatingsection 24 requires a corresponding increase in heat input to the vaporgenerator 1% Flow of high pressure, high temperature fluid from vaporgenerating section 24 to primary superheater 2t: continues through valve56 until the pressure in primary superheater approaches that prevailingin vapor generating section 24*. At such time valve 55 is opened andvalve 42 closed to establish normal operation of the vapor generator andassociated turbine.

FIG. 2 is a representation of a forced flow vapor generating systemwherein the vapor for cooling the superheater 28 and starting turbine 3%is not supplied from an auxiliary source but is produced only byflashing which results from expanding the high pressure liquid leavingprimary superheater 26 and valve 56.

The advantages derived from employing return line 6% in accordance withthe invention are even greater when used in connection with the vaporgenerator illustrativciy shown in FIG. 2, than when used in connectionwith the vapor generator shown in FIG. 1. This is so because in the FIG.2 embodiment the pressure and temperature of the fluid leaving primarysuperheater 26 via return line 66 must be closely watched even duringthe early phase of start-up operation so that steam by flashing can besupplied to the heating surfaces of finishing superheater as early aspossible to protect these surfaces from the heat of the combustion gasessweeping thereover. While steam is thus supplied to the superheater 23via throttle valve 56, turbine valve 31 is closed and by-pass valve 54is controlled so as to gradually raise the pressure and temperature ofthe vapor to desired values suitable for rolling and synchronizingturbine 30. With these considerations in mind the start-up procedure or"the power plant depicted in FIG. 2 proceeds in a like manner as thatearlier herein described in connection with the power plant shown inFIG. 1.

While specific embodiments of the invention have been shown anddescribed, it will be apparent to those skilled in the art that variouschanges, modifications, substitutions, additions and omissions may bemade therein without departing from the spirit and scope of theinvention as set orth in the appended claims.

7 What I claim is:

1. A method of starting up a forced flow modified once- Y through vaporgenerator having a first heatin g section inarranged for flow of avaporizable fluid therethrough; the

continuing the feeding of vaporizable fluid to said first section whilerecirculating said fluid from the outlet of,

said furnace walls to the inlet thereof to maintain the flow velocity insaid furnace walls above a predetermined minimum velocity; permitting asecond overflow from the outlet of said primary superheater to the inletof said furnace walls while discontinuing said first overflow; obtainingan indication of the temperature and pressure of said second over-flow;gradually establishing fiow from said primary superheater to saidfinishing superheater when the temperature and pressure of said secondoverflow reach predetermined values; and re-establishing a flow ofvapor- TlZSJblC fluid to said vapor generator generally proportional tothe flow from saidfinishing superheater.

2. A method of starting up a forced flow modified oncethrough steamgenerator having a first heating section including steam generatingfurnace walls and primary superheater, and having a second heatingsection including a finishing superheater, said heating sections beingserially arranged for flow of water and steam therethrough; theinvention comprising feeding water to said furnace walls and primarysuperheater only; while blocking the flow from said primary superheaterto said finishing superheater heating said water by supplying fuel andair to said steam generator for burning and producing hot combustiongases flowing in heat exchange relation over the heating surfacesthereof including said furnace walls, primary superheater and finishingsuperheater; permitting a first overflow of heated water due toexpansion thereof and for cleaning-up purposes from said first heatingsection to a point of lower pressure; discontinuing the feeding of waterto said first section; while recirculating said water from the outlet ofsaid furnace walls to the inlet thereof to maintain a predetermined:rninimum flow velocity in said furnace walls; permitting a secondoverflow from the outlet of said primary superheater to the inlet ofsaid furnace walls; obtaining an indication of the temperature iandpressure of said secondoverflow; and gradually esta-blishing flow fromsaid primary superheater to saidfinishing superheater when thetemperature and pressure of said second overflow reach predeterminedvalues, and re-establishing a flow of feedwaterto said steam generatorgenerally proportional to the output flow from said steam generator. I i

7 3. A method of starting up a forced flow modified oncethrough steamgenerator having a first heating section including an economizer, steamgenerating furnace walls and primary superheater and having a secondheating section including a finishing superheater said heating sectionsbeing serially arranged for flow of water and steam therethrough; theinventioncomprising feeding water to'said economizer, furnace wallsandprimary superheater-only;

heating said feedwater by supplying fuel and air to said steam generatorfor burning and producing hot combustion gases flowing in *heat exchangerelation over the heat ing surfaces of said furnace walls,primarysuperheater,

finishing superheater and, economizer; permitting a first] overflow ofheated water due to expansion thereof 'and for cleaning-up purposes fromthe outlet of said primary superheater to a point of lower pressure;discontinuing .the feeding of water to said first section whilerecirculating said feedwater from the outlet of said furnace walls tothe to said finishing superheater when the temperatureand pressure ofsaid second overflow has reached predetermined values, andre-esta'blishinga flow of feedwater to said steam generator generallyproportional to the output flow from said steam generator.

4. A method of starting up a forced flow modified oncethrough vaporgenerator having a first heating section 1n-- cluding vapor generatingfurnace walls and a primary superheater, and having a second heatingsection including a finishing superheater, said heating sections beingserially arranged for flow of a vaporizable fluid therethrough; and anauxiliary source of vapor; the invention comprising feeding vaporizablefluid to said first heating section only; feeding vapor from saidauxiliary source to said second heating section; heating saidvaporizable fluid and said vapor by supplying heat to said first andsecond heating sections; permitting a first overflow of heatedvaporizable fluid from said first heating section to a point of lowerpressure; flowing said heated vapor to a point of use; discontinuing thefeeding of vaporizable fluid to said first section while recirculatingsaid fluid from the outlet of said furnace walls to the inlet thereof tomaintain the flow velocity in said furnace walls above a predeterminedminimum velocity; permitting a second overflow from the outlet of saidprimary superheater to the inlet of said furnace walls'while'discontinuing said first overflow; obtaining an indication of thetemperature and pressure of said second overflow; gradually establishingflow from said primary superheater to said finishing superheater whenthe temperature and pressure of said second overflow reach predeterminedvalues while discontinuing the feeding of vapor from said auxiliarysource; and re-esta-blishing a flow of vaporizable fluid to said steamgeneratorgenerally proportional to the output flow from said steamgenerator. 5. In a forced flow modified once-through steamgeneratorhaving a first'heating section including steam generatingfurnace'walls and primary superheater, and having a second heatingsection including a finishing superheater,

means for shutting off flow of water and steam to said second heatingsection; means: for supplying heat 'to said first and secondheatingsections; means for permitting a first overflow of heated waterfrom said first heating section to a point of lower pressure; shut-offmeans for dis-: continuing the feeding of water to saidfirst section;means for recirculating said heated water from the outlet of saidfurnacewalls to the inlet thereof to maintain the flow velocity thereofin said furnace walls above a predeter mined minimum velocity; means forpermitting a second overflow from the outlet of said primary superheaterto the inlet of said furnacewalls; shut-off means for dis continuingsaid first-overflow; means for obtaining an indication of thetemperature and pressure ofsaid-second, overflow; means for graduallyestablishing flow from said primary superheater to said finishingsuperheater when the temperature and pressure of said second overflowreach predetermined values, and means for re-establishing a flow offeedwater to saidsteam generator generally proportional to .the'outputflow therefrom. a

6; In a forced flow modified once-through steam generator having a firstheating section including steam gen erating furnace walls and a primarysuperheater, and having a second heating section including a finishingsuperheater, said heating sections being serially arranged for inletthereof to maintain the flow velocity in said furnace walls above apredetermined minimum velocity; permitting a second overflow from theoutlet of said primary superheater to the inlet 'of said furnace wallswhile dis continuing said first overflow; obtaining an indication of thetemperature and pressure of said second overflow;

gradually establishing flow from said primary superheater flow of waterand steam therethrough; the combination comprising means for feedingwater to said first heating section; means for heating said water bysupplying fuel and air to said steam generator for burning and producinghot combustion gases flowing in heat exchange relation over the heatingsurfaces of said steam generator including said furnace walls, primarysuperheater, and

finishing superheater means for permitting a first overflow of heatedwater due to expansion thereof and for cleaning- 7 up purposes from saidfirst heating section'to a pointof lower pressure; shut-01f means fordiscontinuing the feeding of water to said first section; means" forrecirculat ing said water from the outlet ;of said furnace walls to theinlet thereof to maintain a predeterminedminimum flow I indication ofthe temperature and pressure of said second overflow; means forgradually establishing flow from said primary superheater to saidfinishing superheater when t the temperature and pressure of said secondoverflow reach predetermined values, and means form-establishing a flowof feedwater to said steam generator generally proportional tothe outputflow therefrom.'

g 7. In a forced flow modified once-through steam gen erator having afirst heating'section including an economizer, steam generating furnacewalls,'.primary superheater and a second heating section including afinishing superheater said heating sections being serially arranged forflow of Water and steam'therethrough; the combination comprising meansfor feeding water tosaid first heating section; means for heating saidwater by supplying fuel and air to said steam generator for burning andproducing hot combustion gases flowing in heat exchange relation overthe heating surfaces ofsaid furnace walls; primary superheater,finishing superheater and economizer; means for permitting a firstoverflow of heated water due to expansion thereof and for cleaning-uppurposes from the outlet of said primary superheater to a point of lowerpressure; shut-off means for discontinuing the feeding of Water to 'saidfirst section; means for recirculating said 7 water from the outlet of"said furnace'walls to the inlet thereof to maintain the flow velocitythereof in said furnace walls above a; predetermined minimum velocity;

means for-'permitti-ng'a'second overflow'from the outlet of said primaryIsuperheater to the inlet of "said furnace walls; shut off means fordiscontinuing said first overflow;

' .ineans for obtaining an indication of the temperature and,

pressure of'saidsec'ond overflow;-means'for graduallyestablishing'fiowfrom said'primary superheater to said finishingjsuperheater when the temperature and pressure of said second overflowreach predetermined values,'and

means for re-establishing a'flow of feedwater to said steam generatorgenerally proportional to the output flow there- 8. In .a forced flowmodified once-through steam generator having a first heating sectionincluding steam generating furnace walls and primary superheater, andshaving a second heating section including a finishing superheater,

said heating sections being seriallyarranged for flow of water and steamtherethroug'h; anda sourcebf auxiliary steam; the combination'comprisingmeans for feeding waater to said first heating section; means forshutting ofi flow -I of water and steam to said second heatingsection;means for supplying auxiliary steam to said second heating section;means for supplying heat' to said first and second heating sections;means for-"flowing steam; generated in said steam. generator to a pointof use; means for permitting a'first overflow'of heated'water from saidfirst heating section to a point of lower pressure; shut-off means, fordiscontinuing the feeding of water to said first section; means forrecirculating said heated water from the outlet ing section; and meansfor re-establishing a flow of feedwater to said steam generatorgenerally proportional to the output flow therefrom.

9. In a forced flow modified oncethrough steam generator having a firstheating section including an economizer, steam generating furnace walls,primary superheater and a second heating section including a finishingsuperheater said heating sections being serially arranged for fiow ofwater and steam therethrough; and a source of auxiliary steam, thecombination comprising means for feeding water to said first heatingsection; means for feeding vapor from said auxiliary source to saidsecond heating section; heating-said feedwater to said auxiliary vaporby supplying fuel and air to said steam generator for burning andproducing hot combustion gases flowing in heat exchange relation overthe heating surfaces of said furnace walls, primary superheater,finishing superheater and economizer; means for permitting a firstoverflow of heated water due to expansion thereof and for cleaninguppurposes from the outlet of said primary superheater to a point of lowerpressure; shut-off means for discontinuing the feeding of water to saidfirst section; means for recirculating said feedwater from the outlet ofsaid furnace walls to the inlet thereof tomaintain the flow velocity insaid furnace walls above a predetermined minimum ve'olcity; means forpermitting a second overflow from the outlet of said primary superheaterto the inlet of said furnace walls; shut-off means for discontinuingsaid first overflow; means for obtaining an indication of thetemperature and pressure of said second overflow; means for graduallyestablishing flow from said primary superheater to said finishingsuperheater when the temperature and a, pressure of said second overflowreach predetermined and Control System Integration With the Turbine Gen-1 values; means for discontinuing feeding of vapor from said auxiliarysource; and means for re-esta-blishing a flow of feedwater to said steamgenerator generally proportional to the out'put fiow therefrom.

References Cited in the file of this patent UNITED STATES PATENTS851,784 Great Britain Oct. 19,1960

' OTHER "REFERENCES I Large Sub and SupercriticalSteam Generator Startuperator,. by Charles Strohmeyer, In, reprint of paper preof said furnacewalls to the inlet thereof to maintain the flow velocity thereof in saidfurnace walls above a pre second-overflow from the outlet of'saidprimary superdetermined minimum velocity; means for permitting a Isented at American Power Conference, Chicago, lllinois,

March 1962. 1 V

Mitteilungen Number 1, published September 1956 by DorrwerkeAktiengesellschaft, Ratingen, Germany.

1. A METHOD OF STARTING UP A FORCED FLOW MODIFIED ONCETHROUGH VAPORGENERATOR HAVING A FIRST HEATING SECTION INCLUDING VAPOR GENERATINGFURNACE WALLS AND A PRIMARY SUPERHEATER AND HAVING A SECOND HEATINGSECTION INCLUDING A FINISHING SUPERHEATER SAID HEATING SECTIONS BEINGSERIALLY ARRANGED FOR FLOW OF A VAPORIZABLE FLUID THERETHROUGH; THEINVENTION COMPRISING FEEDING VAPORIZABLE FLUID TO SAID FIRST HEATINGSECTION ONLY; HEATING SAID VAPORIZABLE FLUID BY SUPPLYING HEAT TO SAIDFIRST AND SECOND HEATING SECTIONS; PERMITTING A FIRST OVERFLOW OF HEATEDVAPORIZABLE FLUID FROM SAID FIRST HEATING SECTION TO A POINT OF LOWERPRESSURE; DISCONTINUING THE FEEDING OF VAPORIZABLE FLUID TO SAID FIRSTSECTION WHILE RECIRCULATING SAID FLUID FROM THE OUTLET OF